linux-kernel - Re: [PATCH v6 2/3] gpio: Cygnus: add GPIO driver

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Date:	Tue, 20 Jan 2015 11:17:03 -0800
From:	Ray Jui <rjui@...adcom.com>
To:	Linus Walleij <linus.walleij@...aro.org>
CC:	Rob Herring <robh+dt@...nel.org>, Pawel Moll <pawel.moll@....com>,
	"Mark Rutland" <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"Alexandre Courbot" <gnurou@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	"Christian Daudt" <bcm@...thebug.org>,
	Matt Porter <mporter@...aro.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Joe Perches <joe@...ches.com>, Arnd Bergmann <arnd@...db.de>,
	Scott Branden <sbranden@...adcom.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v6 2/3] gpio: Cygnus: add GPIO driver



On 1/20/2015 1:53 AM, Linus Walleij wrote:
> On Sat, Jan 17, 2015 at 1:11 AM, Ray Jui <rjui@...adcom.com> wrote:
>> On 1/16/2015 2:14 AM, Linus Walleij wrote:
> 
>>> Some hardware designs put the software-controlled biasing
>>> resistors in the GPIO block electronically connected to the actual
>>> pins, so that e.g. the biasing will be available if some MMC or
>>> whatever is using the same pins in another muxing. In such
>>> situations it's quite evident that they need to be a combined
>>> GPIO and pin controller.
>>>
>>> I have some regrets that bolting a second pin controller to the
>>> GPIO chip make things a bit complex but it's a price we have
>>> to pay for getting some kind of generic interface.
>>
>> Okay. In summary, I think both of us think the following approach makes
>> sense in my situation:
>> - leave pinmux in pinctrl-bcm-cygnus.c
>> - leave pinctrl + gpio in pinctrl-bcm-cygnus-gpio.c under drivers/pinctrl/*
>>
>> But by thinking about this more, I thought this would create duplicated
>> pinctrl descriptors in our system, one from the pinmux driver, and the
>> other from this pinctrl+gpio driver. That is probably undesirable?
> 
> No, there are several systems with multiple pin controllers and the
> framework easily handles multiple pin controllers in the same
> system just as well as we handle multiple GPIO chips.
> 
>> By reviewing various drivers in the pinctrl directory, I found what
>> pinctrl-u300.c and pinctrl-coh901.c does seems to serve as a good model
>> for me to follow:
>> - pinctrl-u300.c is the pinmux driver
>> - pinctrl-coh901.c is the gpio+pinctrl driver
> 
> Yeah, I don't know if the separation between them is as beautiful
> as it should be. I used it when developing the pin control
> subsystem.
> 
>> The GPIO pinctrl logic is in the coh901 block, so pinctrl-coh901.c
>> exposed two public functions u300_gpio_config_get, u300_gpio_config_set
>> that pinctrl-u300.c can use. The u300 populates all pinmux/pinctrl
>> related functions into the subsystem. This way there's only one pinctrl
>> descriptor, populated through pinctrl-u300.c.
>>
>> Does that model make more sense to you?
> 
> Yeah I wrote it myself so I'm maybe blind for any dumbness in
> the code. But I think it's kind of elegant. But it is not using the
> generic pinctrl device tree bindings so it's kind of oldstyle and
> bloated in that sense.
> 
> Yours,
> Linus Walleij
> 
Okay. I think I have a pretty good idea of what you expect. Regarding
whether or not to keep pinctrl-bcm-cygnus.c and
pinctrl-bcm-cygnus-gpio.c completely independent with each other and
therefore have two pinctrl in the system, I'll play with it a bit more
and make a decision.

Thanks a lot for spending all these time explaining it to me. Really
appreciate it!
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